The objectives of this project are to characterize possible age-related differences in the in vitro responses of murine lymphohemopoetic cells to mitogens and their differential susceptibility to freezing damage. Because there are optimal cooling velocities for the cryopre-servation of each cell type, T- and B-lymphocytes may be recovered with slight impairment of function using an optimal cooling rate. Changes in cellular structure and/or function with age then may be distinquishable with selective freezing injury to one or both subpopulations. Structures comsidered the sites of freezing damage are the plasma membrane and cellular membrane systems.